X-ray security inspection systems for cargo and shipping containers typically use transmission radiographic techniques. High inspection throughput of cargo and cargo-carrying vehicles is at a premium. Consequently, it is desirable that an entire plane through the cargo be probed simultaneously, and one inspection modality employs a fan-shaped beam to produce images of a target object while the fan beam and detectors are moved relative to the object. Alternatively, the object may be moved in a direction substantially perpendicular to the plane of the fan beam. In cases where illumination is provided by a fan beam of x-ray radiation, useful spatial resolution of contents of the inspected object is typically provided by a plurality of detector elements. The spatial pixel resolution is governed by the dimensions of the detector elements in a plane normal to the propagation direction of the beam, or else by post-collimators limiting the field of view of each detector element.
In cargo imaging applications, it may be necessary for the penetrating radiation to penetrate a significant thickness of highly attenuating material, and a requirement for penetration of more than 300 mm of steel equivalent is not unusual. As used herein, a penetration depth quoted in length of steel equivalent refers to the maximum steel thickness behind which a lead block can still be seen. For thicknesses of steel exceeding the penetration capacity of a particular imaging system, the image will be completely dark, and the block will not be seen.
To ensure the required penetration, inspection systems employed for the inspection of cargo, and in certain industrial applications, typically use x-rays with a maximum energy of several MeV, and, more particularly, in current systems, energies up to about 9 MeV. As used herein and in any appended claims, penetrating radiation of energies of at least 1 MeV may be referred to as hard x-rays or high energy x-rays.
Among non-intrusive inspection methods, x-ray imaging in its many forms is a proven technology capable of detecting a variety of contraband. X-ray systems have been based on transmission imaging in any of a variety of implementations: cone-beam, fanbeam, flying-spot, multi-projection configurations; dual-energy imaging; computed tomography; as well as on imaging incorporating the detection of x-ray radiation scattered in various directions.
Imaging performance is optimized when the incident beam of penetrating radiation, after traversing an inspected object, impinges upon each element of a detector array at as nearly a normal angle as possible to the active detector area of each detector element. In this manner, scatter from one detector element into another and from surrounding structure is minimized, moreover, the spatial resolution obtained by each element is optimized. When each of the elements is disposed at an equal distance from the illuminating source and with an active detector area transverse to the beam, spatial resolution is optimally matched across the field of view of the detector area. This configuration dictates an arcuate arrangement of the detector elements.
In August 2007, the US Congress passed a law, entitled the “Implementing Recommendations of the 9/11 Commission Act of 2007” (Pub. L. 110-53), requiring the screening of all cargo bound for the US prior to loading onto a ship. Standard ocean containers have an outside width of 8′ and height of 8′ 6″. In order to interrogate the contents of the container noninvasively, an irradiating beam of penetrating radiation must traverse each cross section of those dimensions and impinge upon a detector array, dictating a detector array of substantial dimensions.
The use of an x-ray source and an x-ray detector, both located in a portal, for purposes of screening personnel, is the subject, for example, of U.S. Pat. No. 6,094,072, to Smith, issued Jul. 25, 2000, and incorporated herein by reference. A portal, however, is not typically amenable to rapid and flexible deployment, but, rather, requires a dedicated installation. A rapidly relocatable inspection system providing these features is desirable. An L-shaped detector, moreover, as provided by a rectilinear portal, has non-uniform response because detector solid angles in the beam are changing over the angle of the fan beam of penetrating radiation emitted by the source.
The economic impact of the 100%-screening requirement has been analyzed in the literature, typically subject to an assumption that the screening installation cannot be moved.
One configuration particularly well-suited to cargo inspection is provided by subject matter described in US Published Patent Application 2012/00932288, filed Sep. 19, 2011, entitled “Remotely-Aligned Arcuate Detector Array of High Energy X-Ray Imaging,” and incorporated herein by reference.
While the advantages of detectors disposed equidistantly from a source are known, it has been considered impossible to implement an arcuate detector array on a road-capable vehicle because the radius of curvature of such an array, and thus its distance from the source, exceeds the dimensional standards legally applicable to commercial vehicles, particularly those in the U.S. and Europe.
One solution has been to dispose a detector on one conveyance, and to dispose a source on a separate conveyance, as shown in U.S. Pat. No. 7,460,639 (to Tudor et al.). Such a stratagem is less desirable, however, than a solution that would allow both the source and a true arcuate detector array to be conveyed by means of a road-capable vehicle. Such a solution is provided in the present invention.